1. Field of the Invention
The present invention relates to an image processing system for carrying out a pattern matching between two images and a method of processing images, as well as a scanning electron microscope provided with this image processing system.
2. Description of the Related Art
In recent years, in order to control or inspect the manufacturing process of a semiconductor wafer, a liquid crystal panel, an exposure mask thereof, and the like, a scanning electron microscope (hereinafter, referred to as a “SEM”) is increasingly used. The process administrator measures the dimension of a pattern with a specific shape formed at a predetermined position in a wafer or the like, the distance between a plurality of patterns of a specific shape, or the like, using a SEM, and based on these results the process administrator controls a circuit device formed in a wafer or the like, the quality of the manufacturing process, and the like. In addition, the SEM used for such purpose is often called a length measuring SEM.
The inspection by the foregoing length measuring SEM is usually carried out automatically. That is, in an image (hereinafter, referred to as the “observation image” of a sample) of a sample surface, the image being generated in accordance with the amount of electrons emitted or reflected from the sample surface by irradiation of an electron beam, the length measuring SEM detects according to a predetermined pattern matching method a position on the observation image, where the image agrees with a pattern (hereinafter, referred to as a “registered pattern”) of a specific shape being registered in advance (hereinafter, such a position detection is referred to as a “position matching”), and measures the distance between a plurality of registered patterns, for example. At this time, as the registered pattern used for the position matching, a CAD (Computer Aided Design) data created at the time of design is usually employed. Moreover, as the method of a pattern matching, a normalized correlation method or a generalized Hough conversion method is usually used.
Incidentally, in a semiconductor wafer or the like to be inspected, the finer a structure (pattern of a predetermined quality of material) formed on a semiconductor wafer, the more often the structure is not formed like the pattern of the CAD data because of the variations and the like in the manufacturing. For example, a line width in a metal wiring layer may become larger or may become smaller than the line width based on the pattern of the CAD data. Moreover, if a pattern formed in a layer is a rectangular, its corner is usually formed roundly. Furthermore, depending on the quality of material of the formed layer, the observation image of the pattern of this layer by a SEM may look larger or smaller than the actually formed size. Accordingly, the pattern corresponding to the registered pattern in the observation image is often deformed as compared with the registered pattern in the CAD data.
Moreover, in the normalized correlation method or the generalized Hough conversion method, based on the edge extracted from the observation image of a sample and the edge generated from the registered pattern of a CAD data, a pattern matching is usually carried out. In this case, because the edge extracted from the observation image of a sample is often deformed, if a position matching is carried out with the registered pattern of the CAD data, a sufficient positional accuracy may not be obtained or the pattern matching may fail in some cases. One of the reasons resides in the matching algorithm itself of the normalized correlation method or the generalized Hough conversion method.
When a pattern matching between the edges of two images to be inspected is carried out using the normalized correlation method, the normalized correlation method attempts to match even parts of the both edges to each other. For this reason, with the normalized correlation method, the registered pattern is pulled toward the direction of a high degree of coincidence of the edge between the edge of the registered pattern and the edge of the observation image to thereby match with the pattern of the observation image. Moreover, because the direction of a high degree of coincidence of the edge varies depending on the deformation condition of the pattern of the observation image, it will not settle to a uniform direction. Accordingly, a sufficient positional accuracy cannot be obtained in the position matching with the use of the normalized correlation method.
Moreover, the generalized Hough conversion method is also a matching algorithm which attempts to match the edges of two images to each other. In the generalized Hough conversion method, the pattern matching is carried out by carrying out rotation, enlargement and reduction of a pattern, however, for this reason, if the matching is carried out by carrying out enlargement, reduction, and the like to a part of the pattern (portion with a high degree of coincidence), there is inconvenience that the scale fails to agree in other part of the pattern. Moreover, because the scale varies depending on the deformation condition of the pattern of the observation image, it will not settle to a certain value. Accordingly, a sufficient positional accuracy cannot be obtained also in the position matching with the use of the generalized Hough conversion method.
In JP-A-2002-328015 (Paragraph 0053 to Paragraph 0060, FIG. 2 to FIG. 10, corresponding to U.S. Pat. No. 7,026,615), there is disclosed an example of SEM in which one of wafers or the like to be inspected is taken out, and after observing a pattern formed in this wafer or the like by SEM, the pattern obtained from this observation image is re-registered as a registered pattern, and based on this re-registered pattern, a pattern matching is carried out to other wafers or the like to be inspected. In this SEM, not a pattern of a CAD data but a pattern obtained from the observation image of a semiconductor wafer or the like to be inspected, the observation image being observed by this SEM, is used as the registered pattern. Accordingly, because the difference between the shape of the registered pattern and the shape of the pattern to be inspected becomes small, the probability of failing in the pattern matching will decrease. That is, the success rate of the pattern matching can be improved.
However, in case of the pattern matching technique shown in JP-A-2002-328015, an additional work to observe, by SEM, a pattern formed in a wafer or the like to be inspected, and to re-register the pattern obtained from this observation image as a registered pattern is needed. This work can not help but depending on man power and needs to be carried out for each type of product of an integrated circuit or the like formed in a wafer to be inspected. Accordingly, the efficiency of the inspection work, such as a length measurement, will decrease significantly.
Moreover, in JP-A-2002-328015, a matching algorithm such as the normalized correlation method which attempts to match the edges of two images to each other is used. Then, as the registered pattern, the one obtained from the observation image is used. However, in the manufacturing process of a semiconductor or the like, the pattern of a layer formed in a wafer or the like will inevitably have deformation as described above. Accordingly, also in case of JP-A-2002-328015, as long as the matching algorithm which attempts to match the edges of two images to each other is used, the accuracy of a position matching by the pattern matching cannot be improved.